This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We previously showed that disrupting a highly conserved trafficking signal (GYxx[unreadable]) in the Env cytoplasmic tail of SIVmac239 (i.e. deleting Gly-721 and Tyr-721) produces a profoundly attenuated phenotype in rhesus and pigtailed macaques. Following iv inoculation, deltaGY replicates to a high acute viral peak comparable to SIVmac239, but is suppressed to 100 copies/ml with no loss of peripheral CD4 cells. To determine the pathological correlates of attenuation, 4 rhesus macaques were infected iv with deltaGY. Two animals were necropsied on day 28;2 are being followed with serial biopsies of GALT and peripheral lymph nodes. Four animals were inoculated intravaginally to determine deltaGY's competence for mucosal transmission. For iv-infected animals, the acute viral peak was 1.8x10^6 (2.2x10^5-6.0x10^7) with levels for 2 chronically infected animals declining to 1.4x10^3 and 6.4x10^3 by week 16. For ivag-inoculated animals, 1 of 4 became infected (peak= 2.6x10^7) despite 8 weekly inoculations of 350 TCID50. For necropsied animals, abundant virus was detectable in germinal centers of organized lymphoid tissue in intestine and peripheral lymph nodes. However, in striking contrast to SIVmac239- or SIVmac251-infected controls, infection was limited to immune inductive sites (organized lymphoid nodules) and absent from immune effector sites (diffuse lamina propria). There was also little to no depletion of intestinal CD4+/CCR5+ cells in chronic, deltaGY-infected animals (average CD4+/CCR5+= 43.8% of T-cells pre-infection;33.3% wk 22 post-infection). For necropsied animals there was also no detectable virus in CNS despite high acute levels of plasma RNA. Multilabel confocal microscopy of peripheral lymph tissue showed deltaGY in T-cells but not macrophages. Thus, deltaGY replication is driven by infected T-cells in immune inductive sites, but with sparing of macrophages and LPL in immune effector sites. How a defect in Env trafficking disrupts viral spread, and what host immune responses correlate with its control is being investigated.